I just uploard to the Studiotips site, an Excel spreadsheet with the math for
and a couple of jpgs of the diffusers I built on the back wall of my control
room.

The control rooms critical diffusion zone calculated out 155 to 619 Hz. The
diffusion array is designed to work from 161 to 935 (I didn't want to build a
box deeper than 24" so I settled for 161 Hz LF limit). The math sheet will
allow adjustment by changing the bold numbers entered in the sheet for the
modulus depth and/or well width.

I used some pretty cheap boards and just painted them black ... if you want a
nicer look you might opt for furniture grade plywood and do some stain and
finish work.